The blackening of the thumbnail may be related to trauma, infection, onychomycosis and other factors. 1. trauma: for some patients, may be in the labor, accidentally hand thumb nails were smashed or extrusion, and then there will be under the nail bleeding, resulting in under the nails appeared black spot, appear hand thumb nail black symptoms. 2. Infection: Normal nails are relatively soft and brittle, if you do not pay attention to personal hygiene, there may be fungal infections, resulting in black nails, accompanied by flaking and other related symptoms. 3. Nail nevus: Usually there will be longitudinal parallel arrangement of nail black lines, and with the passage of time, gradually widen, increase, so there will be the performance of nail blackening. There may be other reasons for the blackening of thumbnail, if necessary, you need to cooperate with the doctor to do further examination, clear diagnosis and then deal with and treatment.